1. Field of the Invention
The present invention relates to an improved stacked photovoltaic device having an antireflection layer which excels in photovoltaic properties and provides an improved photoelectric conversion efficiency. More particularly, the present invention relates to an improved stacked photovoltaic device comprising a plurality of photovoltaic elements (cells) and including an antireflection layer provided in the interface between two constituent semiconductor layers which excels in photovoltaic properties and provides an improved photoelectric conversion efficiency.
2. Background Of The Invention
As a photovoltaic device for power sources of civilian equipment, solar cells, etc., there has been proposed a photovoltaic device utilizing a pn junction formed by ion spiking or thermally diffusing impurities into a single crystal substrate, for example( made of silicon (Si) or gallium arsenide (GaAs), or formed by epitaxially growing a layer doped with impurities on such a single crystal substrate. However, since the device uses a single crystal substrate as described above, it requires a high temperature process exceeding 1000.degree. C. upon production and it is difficult to fabricate the substrate into a thickness of several hundreds of micrometers or below, reduction of the cost is not easily attained.
A hydrogenated amorphous silicon (hereinafter referred to as "a-Si:H") photovoltaic device proposed by D. E. Carlson in 1976 can be produced by depositing an a-Si:H layer to a thickness of less than 1 .mu.m at a temperature of lower than 300.degree. C. on an inexpensive substrate such as glass and, accordingly, remarkable cost reduction can be expected. In view of this, various researches have been made therefor.
Further, the performance of the device has been improved in recent years, to such an extent of about 10-12% of photoelectric conversion efficiency with a device of small area, as can be applied to solar cells for a power supply.
Further, in the a-Si:H photovoltaic device and the photovoltaic device using an alloy of a-Si:H (they are hereinafter referred to as "a-Si:H system photovoltaic device"), a structure capable of effectively utilizing photon energy in a wide range of wavelength components of sunlight can be attained by successively stacking layers of different optical band gaps (Eg) or conduction types.
The features of the a-Si:H system photovoltaic device will be described below.
As shown in FIG. 5, since sunlight has a peak at a wavelength of about from 500 nm to 600 nm and has components ranging from 300 nm to 1500 nm, it is desirable for a solar cell to absorb wavelength components in a range as wide as possible. By the way, the wavelength region of light that can be absorbed in a solar cell is generally determined by the optical band gap (hereinafter referred to as "Eg") of semiconductors used in the solar cell. Generally, a semiconductor can absorb only light of wavelength shorter than light of energy corresponding to Eg.
Since there is an approximate relationship between the energy h.nu. (unit: eV) and the wavelength .nu. (unit: nm) of the light given as: .lambda.=1240/h.nu., a semiconductor of smaller Eg can utilize light of longer wavelength. For instance, since an a-Si:H semiconductor layer prepared by using monosilane (SiH.sub.4) as the starting material gas by means of a glow discharge decomposition method (GD method) has an Eg of about 1.7 eV, it can absorb sunlight of 300 nm to 730 nm in wavelength. Further, an a-SiGe:H semiconductor film prepared by using SiH.sub.4 gas and germane gas (GeH.sub.4) as the starting material gases by the GD method is advantageous since it has an Eg of about 1.4 eV and can absorb sunlight of 300 nm to 885 nm in wavelength and, accordingly, the number of electron-hole pairs to be excited is great and as a result, more short-circuit photocurrent (hereinafter referred to as "Isc") can be obtained.
On the other hand, since an a-SiC:H film prepared from SiH and CH as the starting material gases by the GD method has an Eg of about 2.0 eV, it can absorb the sunlight only within a range of wavelength from 300 nm to 620 nm and Isc is small, thus there is a difficulty in applying to a solar cell.
By the way, as a factor for determining the efficiency of a solar cell, there is the open circuit voltage (herein-after referred to as "Voc") in addition to the Isc. The value of Voc tends to increase as Eg of the semiconductor layer used (particularly, an i-type layer) becomes greater. For instance, when a non-doped a-Si:H film is used as the i-type layer, the value for 0.7 V when using the above-mentioned non-doped a-SiGe:H film as the i-type layer. Further, if the non-doped a-SiC:H film is used, the value of Voc is about 1.1 V. Accordingly, it is recognized that the voltage is low although the photocurrent is large in the semiconductor film of small Eg. On the other hand, in a semiconductor film of large Eg, although the voltage is high and there is an advantage that the great energy possessed by photons of short wavelength light can be used effectively, it has a drawback of not being capable of absorbing longer wavelength light and the photocurrent is small.
Accordingly, conventional photovoltaic devices still have a drawback that the energy of a wide range of wavelength components in the sunlight can not sufficiently be utilized.
The foregoing is not related only to the a-Si system photovoltaic devices. Similar relationships between Eg and solar cell characteristics can also be recognized in the photovoltaic devices using other kinds of semiconductor materials.
E. D. Jackson proposed an idea for improving the foregoing situation in 1960 (refer to U.S. Pat. No. 2,949,498). In his proposal, a plurality of photovoltaic elements using semiconductor materials of different Eg values are stacked in such an order that the Eg value becomes smaller from the side of incident light and the elements are connected in series. With such a constitution, light of short wavelength (high energy) is absorbed in the photovoltaic elements using semiconductors of large Eg and the energy thereof is effectively outputted as a voltage. On the other hand, light of long wavelength that can not be absorbed in these photovoltaic elements is absorbed in photovoltaic elements using semiconductors of small Eg, whereby the energy of light of wavelength components within a wide range of the sunlight is effectively utilized. However, the proposal employs a complicated structure in which three kinds of photovoltaic elements prepared independently are connected in series by way of wirings and this lacks in practical usefulness in view of the production cost, reliability, etc.
Further, photovoltaic devices in which a plurality of a-Si photovoltaic elements are stacked have been proposed by Japanese patent Laid-open Sho.55(1980)-111180 and Sho.55(1980)-125680. According to these proposals, since any desired number of a-Si:H layers can be stacked as the layers having necessary conduction type and there is no requirement for connecting individual photovoltaic elements in series by means of wirings, practical utility may be obtained to some extent. However, it has not been intended in these proposals to effectively utilize the light energy in terms of Eg for each of the semiconductors of the photovoltaic elements. Further, Japanese Patent Laid-open Sho. 58(1983)-116779 and Sho. 58(1983)-139478 have proposed a-Si:H system photovoltaic devices in which photovoltaic elements of different Eg are stacked, but these proposals are similar to those made by Jackson described above.
None of these photovoltaic devices having a stacked structure (hereinafter referred to as "stacked type photovoltaic device") can sufficiently satisfy the characteristic requirements of a photovoltaic device, particularly, photoelectric conversion efficiency, and there is much room for improvement.